1. Field of the Invention
The present invention relates to a repeater, a wireless communication system, a control method and a recording medium, and more particularly, a repeater which can transmit the same signal received through plural repeaters to a receiving-side wireless communication device without redundancy of the signal, a wireless communication system, a control method and a recording medium.
2. Description of the Related Art
A repeater simultaneously performs transmission (hereinafter, uplink) from a wireless communication device to a repeater and transmission (hereinafter, downlink) from a repeater to a wireless communication device by changing a frequency or a time slot, thereby permitting a wireless communication between wireless communication devices through itself.
A communication area having plural repeater covering areas (hereinafter, repeater areas) can be constituted by connecting plural repeaters together over a network and using those as one wireless communication system. According to such a wireless communication system, in general, a communication channel (frequency in uplinking or downlinking) is set for each repeater. A wireless communication device selects the communication channel of a repeater which constitutes a repeater area where the wireless communication device itself is present. That is, the wireless communication device awaits a calling from another wireless communication device at a downlink frequency of the repeater, and calls another wireless communication device at an uplink frequency of the repeater.
When the wireless communication device calls another wireless communication device via the repeater, the wireless communication device specifies a repeater constituting a repeater area where a destination wireless communication device is present, and causes a repeater constituting a repeater area where the wireless communication device is present to transmit a call activation signal via the communication channel of the repeater.
Traffic increases when there are plural repeaters having the same channel within a range where a radio wave from one wireless communication device is receivable. For example, Unexamined Japanese Patent Application KOKAI Publication No. H05-75514 discloses a technique of detecting a receiving signal level and of discarding the receiving signal when such a level is greater than or equal to a certain level. According to this technique, once data demodulated by a demodulation circuit in a receiving circuit of a wireless repeater station is stored in a control circuit, a carrier sense circuit simultaneously detects a receiving signal level from a demodulated signal from a demodulation circuit. A detected receiving signal level and a predetermined reference signal level are compared with each other, and when it is determined that the receiving signal level is higher than the reference signal level, no transmission order is given to a transmission circuit, and the data stored in the control circuit is discarded. Note that the contents of Unexamined Japanese Patent Application KOKAI Publication No. H05-75514 will be incorporated in this specification by reference.
There is also a problem that a time necessary for relaying becomes longer than necessary as a repeater station holds a signal in order to avoid any signal collision by simultaneous relaying. For example, Unexamined Japanese Patent Application KOKAI Publication No. 2005-20627 discloses a technique of suppressing any collision of frames to be relayed. According to this technique, a wireless repeater station which receives a wireless signal transmitted from a transmitter is used as a wireless relaying procedure starting station, and frames to be relayed are successively transmitted to another wireless repeater station, connected to the wireless relaying procedure starting station via first and second connection units, using wireless signals, while transferring the frames with wired signals. When transferring of the frames to be relayed with wired signals takes a round through all wireless repeater stations in the system and the relayed frames are transmitted from all wireless repeater stations with wireless signals, a wireless relaying procedure is terminated. Note that the contents of Unexamined Japanese Patent Application KOKAI Publication No. 2005-20627 will be incorporated in this specification by reference.
FIG. 13 is a diagram for explaining one specific example of a communication procedure in a conventional wireless communication system. More specifically, communication procedures in a case where two repeaters 2A, 2B having the same channel (frequency) receives a voice signal from the same wireless communication device (handset device) 3A is shown in the figure. The repeaters 2A and 2B individually receive voice signals from the handset device 3A. Next, the repeaters 2A and 2B individually transmit a positional information registration packet (hereinafter, registration packet) including the ID of the handset device 3A (hereinafter, sender ID) and a repeater inquiring packet (hereinafter, inquiring packet) including the ID of a transmission-destination handset device 3D (hereinafter, recipient ID) to a server 1.
As the server 1 receives the registration packet, the server 1 transmits a position registration ACK (acknowledgement) to the repeaters 2A, 2B, respectively. As the server 1 receives the inquiring packet, the server 1 transmits a repeater inquiring respondence packet (hereinafter, respondence packet) including the ID of a repeater 2C (hereinafter, recipient repeater ID) stored correspondingly to the recipient ID in the inquiring packet to the repeaters 2A and 2B, respectively.
The repeaters 2A and 2B individually transmit a voice signal having the same content to the repeater 2C specified by the recipient ID in the respondence packet (procedures P31 and P32). The repeater 2C transmits voice signals redundantly transmitted from the repeaters 2A and 2B, respectively, to the handset device 3D (procedures P33 and P34).
In this fashion, in a wireless communication system provided with plural repeaters 2A and 2B having the same channel (frequency), the plural repeaters 2A and 2B simultaneously receive a voice signal transmitted from the wireless communication device (handset device) 3A, and transmit the voice signal to the handset device 3D via the repeater 2C. When the repeaters 2A to 2C are connected together via wires, the same voice signals are transmitted from the plural repeaters 2A and 2B to the same repeater 2C via a wired circuit. As the repeater 2C receives the same voice signals redundantly, a voice output from the handset device 3D becomes abnormal. For example, when a voice input into the handset device 3A is “a, b, c, d, e”, the handset device 3D outputs a redundant voice like “aa, bb, cc, dd, ee”.
As explained above, when there are plural repeaters 2A and 2B having the same channel (frequency) in plural repeaters, as the repeaters 2A and 2B simultaneously receive voice signals, the same voice signals are redundantly transmitted to the handset device 3D. If all repeaters having the different channels (frequencies) are set, such a phenomenon does not occur, but as a large number of channels are used, intermodulation may occur. Moreover, the number of channels for the repeaters is limited (e.g., total of 27 channels in a specified low power wireless communication), so that it is not possible to use the repeaters simultaneously greater than or equal to the number of channels.
The present invention has been made in view of the foregoing circumstances, and it is an object of the present invention to provide a repeater which can transmit the same signal received through plural repeaters to a receiving-side wireless communication device without redundancy of the signal, a wireless communication system, a control method and a recording medium.